low, blue, and scarlet, half-way down;
passing _then_ gradually to white. Afterwards use lake to darken the
upper half of the vermilion and gamboge; and Prussian blue to darken the
cobalt. You will thus have three more scales, passing from white nearly
to black, through yellow and orange, through sky-blue, and through
scarlet. By mixing the gamboge and Prussian blue you may make another
with green; mixing the cobalt and lake, another with violet; the sepia
alone will make a forcible brown one; and so on, until you have as many
scales as you like, passing from black to white through different
colours. Then, supposing your scales properly gradated and equally
divided, the compartment or degree No. 1. of the grey will represent in
chiaroscuro the No. 1. of all the other colours; No. 2. of grey the No.
2. of the other colours, and so on.

It is only necessary, however, in this matter that you should understand
the principle; for it would never be possible for you to gradate your
scales so truly as to make them practically accurate and serviceable;
and even if you could, unless you had about ten thousand scales, and
were able to change them faster than ever juggler changed cards, you
could not in a day measure the tints on so much as one side of a
frost-bitten apple: but when once you fully understand the principle,
and see how all colours contain as it were a certain quantity of
darkness, or power of dark relief from white--some more, some less; and
how this pitch or power of each may be represented by equivalent values
of grey, you will soon be able to arrive shrewdly at an approximation by
a glance of the eye, without any measuring scale at all.

You must now go on, again with the pen, drawing patterns, and any shapes
of shade that you think pretty, as veinings in marble, or
tortoise-shell, spots in surfaces of shells, &c., as tenderly as you
can, in the darknesses that correspond to their colours; and when you
find you can do this successfully, it is time to begin rounding.

EXERCISE VIII.

Go out into your garden, or into the road, and pick up the first round
or oval stone you can find, not very white, nor very dark; and the
smoother it is the better, only it must not _shine_. Draw your table
near the window, and put the stone, which I will suppose is about the
size of _a_ in Fig. 5. (it had better not be much larger), on a piece of
not very white paper, on the table in front of you. Sit so that the
light may come